The gm-C filter which is a filter comprising a transconductor and a capacitor is a widely used device for reconstructing received signal in a communication apparatus and for anti-aliasing of transmitted signal. The transconductor means a circuit for outputting current proportional to applied input voltage thereto. The output current is calculated with multiplying the applied voltage by transconductance gm.
In a transconductor-capacitor filter, transconductance gm is an importance parameter for determining output current of transconductor and cut-off frequency of a filter. The transconductance gm is determined in accordance with the transconductor.
FIG. 1 shows a circuit diagram of a conventional transconductor.
As shown in FIG. 1, the transconductor comprises first and second NMOS transistors MN 11 and MN 12, first and second bias current sources IB11 and IB12 and a degeneration resistor R11.
The relation between the components will be described with reference to FIG. 1.
The gates of the first and second NMOS transistors MN11 and MN12 form first and second input nodes Vin+ and Vin−, respectively. The drains of the transistors form first and second output nodes Iout1 and Iout2, respectively. Bias current is supplied to the sources of the first and second NMOS transistors MN11 and MN12 from the first and second bias current sources IB11 and IB12. The degeneration resistor R11 is provided between the sources of the first and second NMOS transistors MN11 and MN12.
The first and second input voltage Vin+ and Vin− applied to the gates of the first and second NMOS transistors MN11 and MN12 causes current lo in the first and second output nodes Iout1 and Iout2, which has value calculated with multiplying input voltage Vin by transconductance gm. The transconductance gm is determined by the degeneration resistor R11 and is in inverse proportional to resistance value of R11.
The cut-off frequency of a transconductor-capacitor filter is linearly proportional to gm/C. Thus, transconductance gm of the transconductor maintains an initial value if a passive device R11 having fixed resistance is used as shown in FIG. 1. Further, the bandwidth of the filter has constant value.
Much recent communication system receives more than two signals having different bandwidth one another. If the above-described filter wherein bandwidth is fixed is used for the system, a plurality of filters having different cut-off frequency one another should be used. Thus, a varying bandwidth filter has been studied in order to resolve the problem.
Much recent communication system receives more than two signals having different bandwidth one another. If the above-described filter wherein bandwidth is fixed is used for the system, a plurality of filters having different cut-off frequency one another should be used. Thus, a varying bandwidth filter has been studied in order to resolve the problem.
There is a conventional apparatus which uses MOSFET device for varying cut-off frequency of the filter depending upon received signal, in lieu of degeneration resistor R11. In this apparatus, resistance of the MOSFET device varies by applying different control voltage to the gates of the MOSFET device in accordance with received signal. The resistance of the degeneration resistor can vary in accordance with the received signal, thereby varying transconductance gm of the transconductor.
However, in the conventional art, the range of reluctance that can be obtained from control of voltage applied to the gate of MOSFET device is very limited, thereby limiting the variation of cut-off frequency of the filter. Further, the non-linearity of MOSFET device causes performance deterioration of the filter.
In a transconductance-capacitor filter, the transconductance gm varies up to 50% from design value in accordance with temperature, variation of power voltage and manufacturing process and the like. Thus, the transconductance-capacitor filter should employ tuning circuit that maintains cut-off frequency as being constant.
The conventional tuning circuit is usually an analog tuning circuit that controls transconductance gm of transconductor so as to maintain the cut-off frequency of the filter as being constant.
However, there is problem in that the clock used in the analog tuning circuit causes noise and the circuit operates continuously even when tuing is unnecessary. These are reasons for wasting power and deteriorating filter performance.
U.S. Pat. Nos. 5,245,646 and 5,914,633 disclose a digital tuning circuit for resolving the problems of the analog tuning circuit.
The tuning circuit disclosed by the patents is a tuning circuit of RC active filter which maintains cut-off frequency as being constant with digital control of RC time constant. That is, capacitor of the RC active filter is embodied as capacitor array and digital codes controls on-off of the capacitors of the array so as to compensate the variation of time constant caused by operating condition, temperature and the like.
However, the conventional digital tuning circuit is limited to RC active filter. It is difficult that the circuit is used for tuning circuit of transconductor-capacitor filter which should detect/compensate the variation of transconductance gm.